What is matter consisting of two or more different kinds of atoms called?

Matter that consists of two or more different kinds of atoms is called a compound. Compounds are formed when atoms of different elements chemically bond together, resulting in a substance with unique properties that are different from the individual properties of the elements that compose it. For example, water (H2O) is a compound made up of two hydrogen atoms and one oxygen atom. This chemical combination results in a stable substance with characteristics distinct from either hydrogen or oxygen alone.

Elements, on the other hand, are pure substances that consist of only one kind of atom and cannot be broken down into simpler substances by chemical means. Alloys are mixtures of two or more metals or a metal and another element, but they do not represent a chemical compound as their components retain separate identities within the mixture. Isotopes refer to atoms of the same element that have different numbers of neutrons, resulting in varying atomic masses, rather than being different kinds of atoms.